Control joints are provided in concrete slabs to reduce the amount of cracking due to shrinkage. Control joints depths, placement, and timing place a critical role in the efficiency of a control joint. These key ingredients are discussed in this article
1. The Timing of the Control Joint Cut
Timing is important for an effective cut. Factors such as concrete mix, weather conditions, hardness of concrete and types of machines used for making the control joints determine when to cut the concrete slab.
When the concrete is sawed before it has reached its initial setting time, the aggregates may strew out of the concrete slab and leave a messy, weakened edge along the joint. This effect is known as Ravelling. Transversely sawing too late can result in uncontrolled cracking as the concrete contracts during curing.
The weather condition in which the cuts are made determines the time of making control joints. In hot weather conditions, sawing can be started 4 hours after the concrete is poured. In cold weather conditions, sawing can be started 12 hours after the concrete is poured.
The best way to determine whether the slab is ready is to make trial cuts to check for ravelling. Saw-cutting should start as soon as the ravelling stops during these trial cuts.
2. Location of Control Joint
Control joints should be in the centre of the slab dividing the concrete into equal segments. The spacing of the joints is in the range of 24 to 36 times the thickness of the concrete slab.
If you are using high shrinkage concrete, you might want to decrease the cut spacing.
The factors that determine the location of control joints are :
Continuity: The joints must be continuous.
Square Pattern: They must form segments in square patterns.
Reinforcement: The ideal location of placing the control joints is where the steel reinforcement is less or not found.
Size of the segment: The square segments formed in the hot climatic conditions are larger than conventional segments due to the fast setting of concrete in a big area.
3. Depth of the Control Joint
To determine the depth of the control joint is to cut the joints one-quarter to one-third the slab thickness. If the concrete slab laid is 150mm thick, the depth of the joint must be 40-50mm. If the joint is too deep, aggregate interlocking will not be sufficient to transfer loads. If the saw cut is too shallow, random cracking might occur.
4. How to Cut the Control Joints
The locations where the joints are to be made are calculated and marked using a chalk-line. It must be made sure that all the segments created are of the same size and the cuts are in a straight line. It is recommended to use machines for making the joints rather than using manual means. If water-cutting equipment is used, make sure that the water is running all the way down to the blade. The machine blade depth must be set to the depth of the control joint and the concrete slab must be sawed following the chalk mark. The saw cuts must be cleaned and filler material must be filled in the cuts to protect the reinforcement from corrosion and from a watertight joint.
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